Oscillator stabilizing circuit



June 21, 1938. J. D. REID 2,121,447

OSCILLATOR STABILIZING CIRCUIT Filed Nov. 28, 1936 Patented June 21,1938 UNlTE s OSCILLATOR STABILIZING CIRCUIT John D. Reid, Philadelphia,Pal, assignor to Radio Corporation of America, a corporation of DelawareApplication November 28, 1936, Serial No. 113,117

7 Claims.

v The present invention relates to an oscillator stabilizing circuit forpreventing frequency shift resulting from anode potential variations andvariable load on the oscillator, and has for its primary object toprovide an improved stabilizing circuit of that character in connectionwith an associated converter tube to which the output of the oscillatoris applied.

The invention relates particularly to oscillator circuits for radioreceiving systems in connection with a frequency changer or convertertube, commonly referred to as a first detector. It is, therefore, afurther object of the present invention to provide an oscillatorstabilizing circuit which may be provided in connection with thefrequency changer tube in a superheterodyne receiver.

It is also a still further object of the present invention, to providean improved oscillator stabilizing circuit comprising the combination ofan oscillator tube and a frequency changer tube of the hexode ordouble-control grid type, such as is provided by a tube knowncommercially as an RCA 6L7 tube.

The invention will, however, be better understood from the followingdescription, when con sidered in connection with the accompanyingdrawing, and its scope is indicated by the appended claims.

In the drawing, the figure is a schematic circult diagram of anoscillator circuit provided with stabilizing means in accordance withthe invention.

Referring to the drawing, 5 is an oscillator tube having an oscillatorcircuit 5 particularly adapted for generating ultra high frequencyoscillations and comprising a grid inductance l and .an anode inductance8, with a cathode connection 9 between said inductances. The shuntvariable tuning capacitor is indicated at ill and feed back from theanode is provided by means of :a screen grid III through a by-passcapacitor 112 to the grounded end i3 of the coil 8 and also from themain anode 54 through a feed back winding coupled with the winding 8.

An anode potential is supplied to the screen grid .H and the anode itthrough a supply lead It, the groundconnection being the negative l3supply connection. The control grid of the oscillator device 5 isindicated at ii and is connected in the usual manner with the tunedoscillator circuit through'a grid capacitor is and a grid leak I9. 1

The oscillator device is of the screen grid pentode type in which thesuppresser grid is indicated at 2B, and this is energized from the posi-I tive B supply lead It through a series resistor 2i, forming a filterwith a suitable by-pass capacitor indicated at 22.

A second series resistor is connected in circuit with the suppressergrid 2%, through a lead 23, as indicated ati l. The second resistor isconnected with the positive supply lead It, at a terminal 25. A suitableby-pass capacitor 22?, connected with the lead 23 at the output end ofthe resistor 2d, provides, with the last-named resistor, a second stagefilter for the potential supply of the suppresser grid 28.

In a radio receivingsystem, particularly of the superheterodyne typeemploying a plurality of amplifier tubes, under control of automaticvolume control means, the positive B potential may tend to vary widelyunless the supply means has unusually good regulation, which is seldomthe case. The resulting plate potential variation on the oscillator aswell as load variations of a sociated tubes tends to cause the frequencyto change or shift.

To prevent this tendency to shift frequency, a pentode tube is utilizedas the oscillator of a superheterodyne receiver or the like, and thesuppresser grid of the oscillator tube is supplied with positiveoperating potential underv control of an associated tube which is, inturn, subjected to control of the automatic volume control means.

Since the oscillator and the frequency changer tube or first detectorare closely associated in operation, it is preferable to utilize thefirst detector or frequency changer tube as the control means in theoscillator stabilizing circuit, particularly when the frequency changertube is placed under control of automatic volume control means.

In the present example, a tube adapted for automatic volume controlmeans and as a mixer or frequency changer tube is indicated at and is ofthe type known commercially as an RCA 6L7, having a cathode 3!, a firstor inner control grid 32, a second or outer control grid 33, a screen,grid 34 therefor, and a suppresser grid 35 connected with the anode 3|,together with the usual output anode electrode 36.

The inner grid 32 is connected through a tuned input circuit 36 withincoming signals to be mixed with the oscillator signals, and, throughthis circuit and filter 31, is connected with an automatic volumecontrol lead indicated at 38. As is well known, the automatic volumecontrol circuit receives a controlling potential which varies inaccordance with received signal strength to provide an increasingnegative bias upon the control grids thereby to reduce the gain withincreased signal strength.

The outer or second control grid 33 is connected through a short supplylead 39 with the cathode connection 9 of the oscillator, whereby itreceives energy from the oscillator for mixing with the incomingsignals.

The intermediate or resulting frequency appears in the output circuitindicated at 40, in connection with the output anode 36, and is suppliedto a suitable intermediate frequency or other amplifier (not shown)through a tuned coupling transformer indicated at 4|. The an ode 36receives operating potential through a suitable filter 42 in connectionwith the positive supply lead It at the point 25.

The screen grid 34 is connected with the output end of the first filterresistor 24, which is in series with the suppresser grid 20 of theoscillator, whereby, in common with the suppresser grid, the screen grid34 draws current through the first filter resistor 24. This arrangementpermits variations in the current drawn by the screen grid 3 of themixer or detector 3!], to cause a corresponding variation in thepotential applied to the suppresser grid 29 by reason of the variationin the potential drop established in the common series resistor 2:3.This control is highly effective, because the screen grid current isrelatively high with respect to the suppresser grid current. The screengrid current thus controls as it provides a major portion of the drop inthis common series resistor 24.

With this arrangement, the suppresser grid of the oscillator tube issupplied with positive operating potential in parallel with the screengrid through a potential drop-producing device, the latter alsoproviding a filter stage for the two grids.

When an automatic volume control potential is applied to the controlgrid of the converter or frequency changer tube 30 from the control lead38, the screen voltage on the screen grid 34 rises, which causes thevoltage on the suppresser grid 29 of the oscillator to rise in turn. Theeffect of this rise in voltage on the suppresser grid of the oscillatoris such that it tends to counteract the change in oscillator frequency,which normally takes place because of the change in the loading of thecoupling grid 32 of the converter tube on the oscillator and because ofchanges in the operating potential applied to the oscillator from thesupply source.

I claim as my invention:

1. In a frequency converter system, the combination of an oscillatortube having a suppressor grid, a converter tube having a screen grid, aresistor, and means for supplying positive operating potential to saidgrids in parallel through said resistor as a common series potentialdrop producing circuit element, whereby the potential drop in saidresistor controls the potential on the suppressor grid in accordancewith variations in the screen grid current to said converter tube;

2. In a radio receiver having automatic volume control means, afrequency converter system, the combination of an oscillator tube havinga suppressor grid, a converter tube having a screen grid and having acontrol grid connected with said automatic volume control means, aseries filter resistor, and means for supplying positive operatingpotential to said grids in parallel through said series filter resistoras a common series potential drop producing element, whereby thepotential drop in said resistor controls the potential on the suppressorgrid in accordance with variations in the screen grid current to saidconverter tube through said resistor resulting from operation of theautomatic volume control means, a by-pass capacitor for said filterresistor, and means providing a filter between said grids.

3. In a frequency converter system, the combination of an oscillatortube having a suppressor grid, a converter tube having a screen grid,means for coupling said tubes including a common tuned grid circuit towhich said tubes are connected, whereby the converter tube provides aload on the oscillator tube, means providing a connection between saidsuppressor and screen grids, means for supplying a positive operatingpotential to said interconnected grids including a series potential dropproducing resistor in circiut with said last named grids, and means forapplying a variable biasing potential to the converter tube to controlthe gain therethrough.

4. The combination of an oscillator comprising a tunable oscillatorcircut, an oscillator tube connected therewith for generatingoscillations, said oscillator having a suppressor grid, a tube having acontrol grid and a screen grid, coupled to said oscillator circuit toreceive oscillations therefrom and to provide a variable load thereon inresponse to a variable grid biasing potential, grid means for applying avariable biasing potential to said last-named tube to vary the load onsaid oscillator, and means for minimizing the effect of said variableload in varying the oscillator frequency, said means including aconnection between said screen grid in said last-named tube and saidsuppressor grid, means for supplying positive operating potential tosaid grids in parallel including a common circuit connection,

and a filter in said connection having a series potential drop-producingelement having a resistance of a value sufficiently high for controllingthe potential on the suppressor grid in response to variations incurrent flow to the screen grid.

5. In a frequency converter system, the combination of an oscillatortube having a suppressor grid, a converter tube having a control gridand a screen grid, means for applying a variable';

biasing potential to said control grid of the converter tube, means forcoupling said converter tube to the oscillator tube to receiveoscillations therefrom, said converter tube thereby providing a variableload on the oscillator, and means for energizing said screen andsuppressor grids in parallel at positive potentials including a supplycircuit and a series high resistance element in said circuit providing'apotential drop therein resulting from current fiow to said grids, thecur- 1 rent to the screen grid being relatively high with respect to thesuppressor grid current and'the value of the series resistance providedby said element being such that variations in screen grid current'effectvariations in suppressor grid'potential.

6. In a radio receiving system, the combination of an oscillator tubehaving a suppressor grid, a mixer tube having two control gridsand ascreen grid, means for coupling one of said control grids to theoscillator, means for applying volume control potentials and received.signals to the other of said control grids, a circuit for applyingpositive operating potentials tothe screen grid including a seriespotential drop- "78 producing resistor, and means for applying apositive operating potential to the suppressor grid of the oscillatorcomprising a connection between said suppressor grid and the screen gridof mixer tube, whereby said screen and suppressor grids are parallel-fedthrough said potential drop-producing resistor.

7. In a superheterodyne receiver, the combination of a pentodeoscillator tube, oscillation generating circuits connected with saidtube, means for tuning said oscillation generating circuits through apredetermined frequency range, a frequency converter tube for convertingsaid oscillations and received signal oscillations into a predeterminedintermediate frequency signal,

said frequency converter tube having a pair of control grids, a screengrid and an output anodel electrode for intermediate frequency signals,

means providing a signal-conveying connection between one of saidcontrol grids and one of the oscillation generating circuits forsupplying oscillations therefrom to said converter tube, a tunablesignal input circuit connected with the other of said control grids,means for applying automatic volume control potential to said lastnamedgrid, and means for jointly supplying positive operating potential tothe screen grid of the converter tube and to the suppressor grid of theoscillator including a potential drop-producing resistor through whichscreen and suppressor grid currents flow, whereby the suppressor gridpotential is caused to rise in response to an increase in potential onthe screen grid of the converter tube following an increase in automaticvolume control potential on the signal input control grid of saidconverter tube, thereby to compensate for the effect of load change onthe oscillator by reason of variation-in automatic volume controlpotential.

' JOHN D. REID.

